Chapter 17: Linked Lists Starting Out with C++ Early Objects Eighth Edition by Tony Gaddis, Judy Walters, and Godfrey Muganda

Topics 17.1 Introduction to the Linked List ADT 17.2 Linked List Operations 17.3 A Linked List Template 17.4 Recursive Linked List Operations 17.5 Variations of the Linked List 17.6 The STL list Container

17.1 Introduction to the Linked List ADT Linked list: a sequence of data structures (nodes) with each node containing a pointer to its successor The last node in the list has its successor pointer set to NULL NULL list head

Linked List Terminology The node at the beginning is called the head of the list The entire list is identified by the pointer to the head node. This pointer is called the list head.

Linked Lists Nodes can be added or removed from the linked list during execution Addition or removal of nodes can take place at beginning, end, or middle of the list list head NULL Add or delete node

Linked Lists vs. Arrays Linked lists can grow and shrink as needed, unlike arrays, which have a fixed size NULL list head

Node Organization A node contains: data: one or more data fields – may be organized as structure, object, etc. a pointer that can point to another node pointer data

Empty List A list with no nodes is called the empty list In this case the list head is set to NULL NULL list head

Creating an Empty List Define a pointer for the head of the list: ListNode *head = NULL; Head pointer initialized to NULL to indicate an empty list See pr17-02.cpp head NULL

C++ Implementation Implementation of nodes requires a structure containing a pointer to a structure of the same type (a self-referential data structure): struct ListNode { int data; ListNode *next; }; See pr17-01.cpp

C++ Implementation Nodes can be equipped with constructors: struct ListNode { int data; ListNode *next; ListNode(int d, ListNode* p=NULL) {data = d; next = p;} };

Building a List from a File of Numbers ListNode *head = NULL; int val; while (inFile >> val) { // add new nodes at the head head = new ListNode(val, head); };

Traversing a Linked List List traversals visit each node in a linked list to display contents, validate data, etc. Basic process of traversal: set a pointer to the head pointer while pointer is not NULL process data set pointer to the successor of the current node end while

Traversing a Linked List nodePtr 5 13 19 NULL list head nodePtr points to the node containing 5, then the node containing 13, then the node containing 19, then points to NULL, and the list traversal stops

17.2 Linked List Operations Basic operations: add a node to the end of the list insert a node within the list traverse the linked list Delete/remove a node from the list delete/destroy the list

Creating a Node ListNode *p; int num = 23; p = new ListNode(num); 23 p NULL 23

Appending an Item To add an item to the end of the list: If the list is empty, set head to a new node containing the item head = new ListNode(num); If the list is not empty, move a pointer p to the last node, then add a new node containing the item p->next = new ListNode(num);

Appending an Item p 5 13 23 list head NULL list head See NumberList.h, NumberList.cpp, and pr17-03.cpp List originally has nodes with 5 and 13. p locates the last node, then a node with a new item, 23, is added

Destroying a Linked List Must remove all nodes used in the list To do this, use list traversal to visit each node For each node, Unlink the node from the list Free the node’s memory Finally, set the list head to NULL

Inserting a Node Used to insert an item into a sorted list, keeping the list sorted. Two possibilities: Insertion is at the head of the list (because item at head is already greater than item being inserted, or because list is empty Insertion is after an existing node in a non-empty list

Inserting a Node at Head of a List Test to see if head pointer is NULL, or node value pointed at by head is greater than value to be inserted Must test in this order: unpredictable results if second test is attempted on an empty list Create new node, set its next pointer to head, then point head to it

Inserting a Node in Body of a List Requires two pointers to traverse the list: pointer to locate the node with data value greater than that of node to be inserted pointer to 'trail behind' one node, to point to node before point of insertion New node is inserted between the nodes pointed at by these pointers

Inserting a Node into a Linked List previousNode nodePtr 5 13 19 NULL list head num 17 Item to insert Correct position located

Inserting a Node into a Linked List previousNode nodePtr 5 13 19 NULL list head 17 See SortedNumberList.h, SortedNumberList.cpp, and Pr17-04.cpp New node created and inserted in order in the linked list

Removing an Element Used to remove a node from a linked list Requires two pointers: one to locate the node to be deleted, one to point to the node before the node to be deleted

Deleting a Node Contents of node to be deleted: 13 previousNode nodePtr 5 13 19 NULL list head Locating the node containing 13

Deleting a Node previousNode nodePtr 5 13 19 list head NULL list head Adjusting pointer around the node to be deleted

Deleting a Node previousNode nodePtr 5 19 list head NULL list head See NumberList.h, NumberList.cpp, pr17-05.cpp Linked list after deleting the node containing 13

17.3 A Linked List Template A linked list template can be written by replacing the type of the data in the node with a type parameter, say T. If defining the linked list as a class template, then all member functions must be function templates Implementation assumes use with data types that support comparison: == and <= See LinkedList.h, pr17-06.cpp

17.4 Recursive Linked List Operations A non-empty linked list consists of a head node followed by the rest of the nodes The rest of the nodes form a linked list that is called the tail of the original list

Recursive Linked List Operations Many linked list operations can be broken down into the smaller problems of processing the head of the list and then recursively operating on the tail of the list

Recursive Linked List Operations To find the length (number of elements) of a list If the list is empty, the length is 0 (base case) If the list is not empty, find the length of the tail and then add 1 to obtain the length of the original list

Recursive Linked List Operations To find the length of a list: int length(ListNode *myList) { if (myList == NULL) return 0; else return 1 + length(myList->next); }

Recursive Linked List Operations Using recursion to display a list: void displayList(ListNode *myList) { if (myList != NULL) cout << myList->data << " "; displayList(myList->next); } See pr17-07.cpp

Other Recursive Linked List Operations Insert and remove operations can be written to use recursion General design considerations: Base case is often when the list is empty Recursive case often involves the use of the tail of the list (i.e., the list without the head). Since the tail has one fewer entry than the list that was passed in to this call, the recursion eventually stops. See NumberList2.cpp, NumberList2.h, and pr17-08.cpp

17.5 Variations of the Linked List Other linked list organizations: doubly-linked list: each node contains two pointers: one to the next node in the list, one to the previous node in the list 5 13 19 NULL list head NULL

Variations of the Linked List Other linked list organizations: circular linked list: the last node in the list points back to the first node in the list, not to NULL 5 13 19 list head

17.6 The STL list Container Template for a doubly linked list Member functions for locating beginning, end of list: front, back, end adding elements to the list: insert, merge, push_back, push_front removing elements from the list: erase, pop_back, pop_front, unique See pr17-09.cpp

Chapter 17: Linked Lists Starting Out with C++ Early Objects Eighth Edition by Tony Gaddis, Judy Walters, and Godfrey Muganda